Natural killer (NK) cells are a sub-population of lymphocytes that are involved in non-conventional immunity. Characteristics and biological properties of NK cells include the expression of surface antigens such as CD16, CD56 and/or CD57, and the absence of the alpha/beta or gamma/delta TCR complex expressed on the cell surface; the ability to bind to and kill cells that fail to express “self” MHC/HLA antigens by the activation of specific cytolytic enzymes; the ability to kill tumor cells or other diseased cells that express a NK activating receptor-ligand; and the ability to release protein molecules called cytokines that stimulate or inhibit the immune response.
NK cell activity is regulated by a complex mechanism that involves both activating and inhibitory signals. Several distinct classes of NK-specific receptors have been identified that play an important role in the NK cell mediated recognition and killing of HLA Class I deficient target cells. One such class of receptors, the NCRs (for Natural Cytotoxicity Receptors), includes NKp30, NKp46 and NKp44, all members of the Ig superfamily. Their cross-linking, induced by specific mAbs, strongly activates NK cells, resulting in increased intracellular Ca++ levels, triggering of cytotoxicity, and lymphokine release.
NK cells are negatively regulated by major histocompatibility complex (MHC) class I-specific inhibitory receptors. These specific receptors bind to polymorphic determinants of MHC class I molecules or HLA present on other cells and inhibit NK cell lysis. In humans, certain members of a family of receptors termed killer Ig-like receptors (KIRs) recognize groups of HLA class I alleles.
NK cell population or clones that are KIR mismatched, i.e., population of NK cells that express KIR that are not compatible with a HLA molecules of a host, have been shown to be the most likely mediators of the graft anti-leukemia effect seen in allogeneic transplantation (Ruggeri, L., et al, Science 295, 2097-2100). One way of reproducing this effect in a given individual would be to use reagents that block interactions between inhibitory receptors on NK cells and their ligands which impart a protection from NK cell mediated lysis. Accordingly, practical and effective approaches in the modulation of NK cell activity would be of great value. Such an approach would be useful in the treatment of a wide range of diseases.
Certain proliferative diseases such as cancers lack effective treatment. Neuroblastomas, carcinomas and melanomas are particularly notable examples of such cancers. Neuroblastoma has a particularly poor prognosis. Neuroblastoma, the most common solid tumor of childhood, can arise anywhere along the sympathetic nervous system (Brodeur, G.-M. Neuroblastoma. (2003) Nat Rev Cancer. 3, 203-216; Schwab, M., Westermann, F., Hero, B., & Berthold, F. (2003) Lancet Oncol. 4, 472-480). Frequently, the primary tumor localizes in the abdomen with the adrenal gland being the most common site. The age of 1 year represents an important prognostic cut-off. Under 1 year of age most tumors are localized (stages 1-2) or, if disseminated, undergo maturation into benign ganglioneuromas or regress spontaneously (the so called stage 4S). On the contrary, most children of more than 1 year of age present with a highly disseminated disease at diagnosis with metastasis involving bone marrow (BM), brain, liver and skin (stage 4) and a very poor prognosis. Indeed, neuroblastoma is the tumor with the highest risk of death in children. In particular, due to drug resistance or relapse after conventional therapy, children at stage 4 have very poor survival rates (3 years probability survival <15%). One of the aims of the current research is to improve our understanding of the biological behavior of neuroblastoma and to identify novel markers that would be used for diagnosis, monitoring of the disease as well as for attempting innovative therapeutic approaches. In particular, the characterization of surface molecules expressed by neuroblastoma cells would allow a more precise identification and quantification of tumor cells, particularly in the BM, a frequent site of tumor relapses. This will improve the diagnosis and allow to better defining the risk grade in a particular patient as well as to deliver the most appropriate therapy. In this context, the disialoganglioside GD2 is generally used as a neuroblastoma-associated marker (Schulz, G., Cheresh, D.-A., Varki, N.-M., Yu, A., Staffileno, L.-K., & Reisfeld R.-A. (1984) Cancer Res. 44, 5914-5920; Hakomori, S. (1984) Annu. Rev. Immunol. 2, 103-126). However, anti-GD2-monoclonal antibodies (mAb) also react with cells other than tumor cells. A conceivable explanation is that GD2 shed from the cell surface of neuroblastoma may bind to the surface of other cells and react with anti-GD2 specific mAb (Ladish, S., Wu, Z.-L, Feig, S., Schwartz, E., Floutsis, G., Wiley, F., Lenarsky, C., & Seeger, R. (1987) Int. J. Cancer. 39, 73-76; Valentino, L., Moss, T., Olson, E., Wang, H.-J., Elashoff, R., & Ladisch, S. (1990) Blood 75, 1564-1567). Identification of novel surface antigens expressed by neuroblastoma would also allow to selectively isolate fresh tumor cells that could be assessed for their susceptibility to NK-mediated lysis. Indeed, in vitro-cultured neuroblastoma cell lines have been shown to be susceptible to NK-mediated lysis (Sivori, S., Parolini, S., Marcenaro, E., Castriconi, R., Pende, D., Millo, R., & Moretta, A. (2000) J. Neuroimmunol. 107:220-225). There is therefore a need in the art for novel surface antigens expressed on cancer cells.